Optical telecommunications service providers continue to demand more data capacity and higher data transmission speeds to service their customers' current and future requirements. In systems employing optical amplifiers and, particularly, EDFAs, channel density has been limited by the usable gain bandwidth of the EDFA. This bandwidth is on the order of 35 nm even when gain flattening filters are used to flattened the erbium gain spectrum for multichannel amplification. Three techniques for increasing system capacity in multichannel lightwave systems include (1) increasing bit rate per channel, (2) increasing the number of channels by decreasing channel spacing, and (3) increasing number of channels by increasing the total gain/transmission bandwidth of the gain media. Increasing the bit rate per channel is not always a viable solution as many installed systems cannot operate beyond the current OC-48 bit rate (2.5 Gb/s). Likewise, fiber nonlinearities limit reducing channel spacing below the current values of 50 GHz-100 GHz. Thus increasing the EDFA gain bandwidth allows a direct path for increasing system capacity while maintaining channel spacing and bit rate per channel. As far back as 1990, Ainslie et al., High gain, broadband 1.6 micron ER.sup.3+ doped silica fiber amplifier, Electronics Letters, volume 26, pp. 1645-1646 (1990) investigated the long band (1565-1610 nm) utility of the erbium gain spectrum. Recently, Srivastava et al., 1 Tb/s transmission of 100 WDM 10 Gb/s channels over 400 km of Truewave fiber, Tech. Dig. OFC'98, Post deadline paper PD10-1, San Jose, Calif., 1998, demonstrated application of silica EDFAs in the 1.6 micron band (L-band). Sun et al., Ultrawide band erbium-doped silica fiber amplifier with 80 nm of bandwidth, PROC. OAA, Post deadline paper PD 2-2, Victoria, BC Canada, 1997, discussed a split band architecture that amplifies both the conventional C-band (1530 nm-1560 nm) and the L-band, providing a total gain bandwidth of 80 nm. Thus L-band amplification offers a demonstrable, but undeveloped, solution to the bandwidth constraints in WDM lightwave systems.
It is appreciated by those skilled in the art that EDFAs operating in the L-band (herein defined as the spectral range from about 1560-1615+ nm) typically have features that distinguish them from amplifiers designed to work in the heavily used C-band from approximately 1530 nm to 1560 nm. Among the notable differences is a relatively flatter gain spectra at low inversions (i.e., 0.4 versus 0.6-0.7), which necessitates lengths of erbium doped fiber on the order of greater than or equal to about 75 m up to about 300 m (for current typical Er concentrations), in contrast to less than or equal to about 50 m for conventional C-band devices. This is due at least in part to the relatively low emission cross section of erbium for wavelengths greater than approximately 1560 nm. A consequence of the unusually long erbium doped fiber lengths required for low inversion amplification is the generation of large amounts of reverse traveling ASE. Moreover, the unique L-band operating environment impacts the choice of pump wavelength in the 980 nm absorption band. In any event, EDFA L-band amplifiers are an essential enabling technology for systems operating in what is now referred to as the 4.sup.th generation telecommunications window.
Pump wavelength detuning in the 980 nm band for C-band amplification, primarily intended towards relaxing the wavelength constraints on 980 nm semiconductor pump laser diodes, has been reported on by a number of authors. Pederson et al., Gain and Noise Penalty for Detuned 980 nm Pumping of Erbium-Doped Fiber Power Amplifiers, IEEE Photonics Technology Letters, Vol. 4, No. 4, pp. 351-353 (April 1992) measured small-signal gain and noise performance of erbium-doped fiber power amplifiers pumped in the 980 nm band as a function of fiber length and pump wavelength. Their findings, in part, showed that signal output power for a C-band input signal at 1551 nm decreased as the pump wavelength was detuned .+-.20 nm from the 979 nm absorption peak. Pederson et al., Gain and Noise Properties of Small-Signal Erbium-Doped Fiber Amplifiers Pumped in the 980-mn Band, IEEE Photonics Technology Letters, Vol. 4, No. 6, pp. 556-558 (June 1992) examined the effects of pump-wavelength detuning on small-signal EDFA's for C-band input signals at 1532 mn and 1551 nm (see FIG. 2 therein). Percival et al., Erbium-Doped Fibre Amplifier With Constant Gain For Pump Wavelengths Between 966 and 1004 nm, Electronics Letters, Vol. 27, No. 14, pp. 1266-1268 (July 1991) reported constant gain for a C-band input signal at 1536 nm over a 38 nm pump range centered at 980 nm provided the fiber had a correct cut-off wavelength.
In contrast to the reported work, the inventors now describe the heretofore unappreciated benefits of 980 nm pump detuning for L-band amplification in the condition of large-signal input power. These benefits include L-band gain and improved pump-to-signal conversion efficiency. As used herein, large-signal input a power refers to input signal conditions in the presence of which the optical amplifier operates in saturation and yields an output signal power which essentially does not depend upon input signal power, but rather depends solely on the pump power, such that P.sub.out =KP.sub.pump, where K essentially represents the efficiency of the amplifier. In contrast, small-signal amplification provides that the output signal power is proportional to the input signal power via the amplification or gain of the amplifier such that P.sub.out =GP.sub.in. These are terms well understood by those skilled in the art.